A conventional ventilation fan of this kind includes a body case having an indoor air suction opening defined in a lower wall thereof and an indoor air discharge opening defined in a side wall or upper wall thereof, a fan mounted in the body case, and a controller for controlling the operation of the fan. Further, for example, a human body detecting sensor is mounted as a sensor for detecting an indoor environment and connected to the controller. If the presence of a human body is detected by the human body detecting sensor, the controller controls the fan to drive it or increase the air volume (see, for example, Patent Document 1 or 2).